1. Field of the Invention
The present invention relates to a paste composition having various characteristics, and a circuit board using the same.
Further, the present invention relates to a ceramic green sheet having insulating, dielectric and magnetic properties and a ceramic substrate using the ceramic green sheet, and a method for manufacturing a ceramic multilayer substrate.
2. Description of the Related Art
In order to allow the circuit board integrated into electronic appliances such as mobile terminal devices and computers to exhibit its function, conductor patterns and via-holes should be formed through the substrates. The conductor patterns and via-holes in the circuit board mounted in these electronic appliances should be fine and highly precise, especially in order to comply with the recent requirements for making high frequency electronic devices to be highly integrated and to respond to high speed signals.
The via-holes have been usually formed by printing an insulating paste composition prepared by mixing an insulating inorganic powder such as a glass with an organic vehicle through a given screen mask on the substrate, and filling a conductive paste composition into the holes intended to become the via-holes, followed by firing the substrate. However, a via-hole having a diameter of at best approximately 200 .mu.m can be formed due to blotting and blurring during the screen printing.
Japanese Unexamined Patent Publication No. 6-283846 discloses, on the other hand, a method for forming the fine via-holes by a photolithographic method using a photosensitive paste composition prepared by additionally mixing photosensitive organic components in the insulating paste composition.
Recently, it has been desired that the photosensitive paste composition used in the photolithographic method be able to be developed with water or an aqueous alkaline solution in consideration of the effect on the environment. Therefore, acidic functional groups such as carboxyl groups are introduced into the photosensitive organic binder.
However, when multivalent metal compounds (especially multivalent metal oxides) having a valence of two or more are used as the insulating inorganic powder in the photosensitive organic binder, the multivalent inorganic ions dissolved into the soluble fraction of the photosensitive paste composition react with anions of the photosensitive organic binder, forming three dimensional network of ionic cross-links to develop gelation. Coating of the paste becomes difficult when the photosensitive paste composition is gelled or, even if coating be possible, the development process becomes so unstable that the paste composition is hardly applicable.
For preventing gelation, Japanese Unexamined Patent Publication No. 9-218509, 9-218508 and 9-222723 have disclosed photolithographic methods using photosensitive paste compositions comprising phosphorous containing compounds such as phosphoric acid, compounds having azole structures such as benzotriazole, and organic compounds having carboxyl groups, respectively. However, the time interval before forming a gel of the photosensitive paste composition is merely extended for a while by these methods, and using the photosensitive paste composition comprising an inorganic powder containing multivalent metal compounds such as a glass powder and ceramic powder have been practically difficult.
Japanese Unexamined Patent Publication No. 10-171107 discloses a method for preventing gelation, thus improving preservation stability, by including 3-methyl-3-methoxybutanol in the photosensitive paste composition. However, since 3-methyl-3-methoxybutanol has a boiling point as low as 174.degree. C., the compound is completely evaporated from coating films that have been dried after coating, often decreasing the effect for preventing gelation. It is also a problem that the portions not exposed to the light are hardly dissolved into the development solvent when the photosensitive paste composition using the compound is subjected to a development treatment.
Further, it is effective for making ceramic multilayer substrates represented by substrates for use in high frequency modules and hybrid ICs to be more multifunctional and highly integrated, and to have higher performance, to form a high density wiring while packaging high precision passive members in the ceramic multilayer substrate. For example, Japanese Unexamined Patent Publication No. 9-92983 discloses a method for manufacturing a ceramic multilayer substrate integrating passive members such as inductors and capacitors, wherein the inductors and capacitors are partially formed on the ceramic multilayer substrate using a ceramic green sheet having various characteristics such as an insulating property, a dielectric property and a magnetic property.
Fine via-holes should be formed through the ceramic green sheet having the insulating property, dielectric property and magnetic property, in order to make the ceramic multilayer capacitor to be multifunctional and highly integrated, and to have higher performance. Via-holes have been formed through the ceramic green sheet by punching with a press mold. However, since the ceramic green sheet is processed by impressing a mechanical load, a via-hole having a diameter of at most approximately 100 .mu.m can be formed.
Japanese Unexamined Patent Publication No. 10-279364 discloses, for example, a method for forming finer via-holes using a photolithographic method. This method comprises preparing a photosensitive ceramic green sheet by forming a sheet of a slurry by mixing a photosensitive resin and ceramic powder, and exposing the green sheet with an activation light beam such as UV light through a photo-mask followed by development for forming fine via-holes.
However, the organic polymer compound in organic binders, especially in photosensitive organic binders, has an acidic functional group that liberates protons such as a carboxylic group and hydroxyl group as its side chain. When a powder containing multivalent metals or their oxides such as a glass powder and ceramic powder is mixed with an organic polymer compound having acidic functional groups, ionic cross links among the multivalent metal ions dissolved out of the powder and organic polymer compounds are formed to cause gelation by forming a three dimensional network.
A nonuniform slurry in which gels are formed has a low processing performance, making it difficult to form a sheet as well as via-holes and conductor patterns. Forming a ceramic green sheet using a nonuniform slurry results in a mechanically fragile ceramic substrate due to, for example, cracks generated during firing.